Wow, he seems awfully little for a delayed reward... I do marbles for good things (and I take them away for not so good things) like practicing piano without being asked, being respectful, helpful, etc. and then they can turn in their marbles for screen time. But I do it with my 8 yo and 5 yo, and he really didn't get into it until fairly recently. Maybe 4-ish. I highly recommend it in general, because it still works with my 8yo most of the time, after at least 2 years of marbles!
With potty, we did do treats, I think they got 1 chocolate chip each time or something. I got a little worried for a while that it would be forever and they'd be asking for chocolate chips when they were 12..., but they eventually forget about the treat and just go potty for potty's sake. I don't think it's terrible.
I agree not to make a big deal out of it.